Request for Comments (=RFC) document 1889 describes the RTP protocol and the RTCP protocol (RTP=Real-Time Transport Protocol; RTCP=Real-Time Transport Control Protocol). RTP provides end-to-end network transport functions suitable for applications transmitting real-time data, such as audio, video or simulation data, over multicast or unicast network services.
Transmission over and within packet-switched networks is characterised by a strongly varying, sometimes very intense, i.e., bursty, data traffic. Therefore, real-time transmissions using IP are prone to face several problems, especially in the case of bandwidth limited home equipment and access equipment. The bursty nature of signalling and control traffic impacts the real-time traffic, only partly diminished by the existing simple multiplexing mechanisms. The signalling information needs not be real-time but a minimum transfer guarantee is required to ensure connectivity with the signalling entity on the remote side.
The bursty nature of signalling does not allow to treat the signalling with the same priority as the real-time streams. On the other side, the treatment of signalling with lower priority may lead to disruption in signalling connectivity. For limited access bandwidth it could occur that, e.g., an existing prioritized real-time connection inhibits the signalling traffic for releasing the call.
Firewalls using NAT/NAPT used in home environment make it difficult for end-to-end signalling protocols like, e.g., SIP, to bind the correct media related streams together (NAT=Network Address Translation; NAPT=Network Address Port Translation; SIP=Session Initiation Protocol). A general solution working with all kinds of such NAT/NAPT devices does not exist. Moreover, SIP contains also transport addresses which need to be corrected when a firewall traversal has occurred. RTP termination occurs usually on a different system than the SIP treatment. So, additional control extensions are needed to transmit the information needed to modify the SIP messages. Moreover, a user is forced to open many ports for VoIP if he uses a firewall using NAT/NAPT (VoIP=Voice over Internet Protocol).